Add a "Discover from playlists" tab to the Channel manager that lists
channels appearing in the user's playlists they don't subscribe to, with
a one-click Subscribe.
- GET /api/channels/discovery: local join (playlist_items -> videos ->
channels) minus the user's subscriptions and their own channel. Enriches
stub channels' metadata up front (title/thumbnail/subscriber count via the
API key) so the user can judge a channel before subscribing; videos are
not pulled (the scheduler picks those up).
- POST /api/channels/{id}/subscribe: write-scope gated, subscriptions.insert
+ local Subscription with the returned resource id.
- YouTubeClient.insert_subscription / get_my_channel_id.
- users.yt_channel_id (migration 0019) caches the user's own channel id so
discovery can exclude it.
- Frontend: ChannelDiscovery DataTable, Channels tab toggle (persisted),
api methods, trilingual strings. Subscribe ships a typed notification
payload (ChannelSubscribedMeta) for the inbox to act on.
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